Cellular radio communication systems typically include a number of central communication base sites. Each central communication site has a service area coverage for servicing mobile communication units within the service area. The service areas typically are arranged such that adjacent remote base site service coverage areas overlap in a manner that provides a substantially continuous service region. The substantially continuous service region provides uninterrupted service by handing off mobile communication units from one base site serving a service area to an adjacent base site serving another service area.
Pedestrian as well as mobile users will typically access the same cellular radio communication systems. For purposes of this discussion, a pedestrian user is one who walks or roams slowly (traveling at 10 kilometers per hour (kph) or less) as opposed to a mobile user who rides in a vehicle (traveling up to 100 kph or more). However, these cellular communication systems are typically designed to provide adequate performance for the worst case environment (i.e., the mobile user). As such, the cellular radio communication systems typically provide continual overhead measurements used by the system to maintain channel quality or perform hand-off functions. Since these measurements require the same amount of processing whether a user is a mobile user or a pedestrian user, the pedestrian user is charged at the same air-time rate for using their cellular phone as the user who is a mobile user.
Therefore, there exists a need in the industry for a personal communication system (PCS) which would provide a low tier system for pedestrian users at a reduced cost. The low tier system would provide access via radio frequency (RF) link to a basic cellular network which may or may not provide hand-off capability between low tier service areas. In addition, a high tier system should be provided for the mobile user. This high tier system, unlike the low tier system, would have many of the features found in current cellular systems including hand-off between high tier service areas and high quality error protection.
Both the high tier and the low tier systems share some basic design constraints. One such constraint is that both systems require initial synchronization of signals between a central communication base site and a mobile communication unit which are communicating, via a radio communication link, with each other. This initial synchronization is particularly important in the high tier system, because this system must be designed to handle synchronization even though the mobile communication unit may be traveling at 100 kph. Synchronization of the two communication devices is necessary to allow the originally transmitted signal to be quickly and easily recovered from a received signal.
In communication systems, with respect to synchronization, two general areas of uncertainty of the signal exist which must be resolved before a received signal can be recovered. These areas of uncertainty are phase and frequency of the carder. In addition, the clock rate can be a source of synchronization uncertainty. Most of this uncertainty may be eliminated by utilizing accurate frequency sources in both communication devices which are communicating with each other. However, some uncertainty can not be eliminated by the use of accurate frequency sources. Doppler-related frequency errors typically can not be predicted and will affect the carder frequency. The amount of Doppler-related frequency uncertainty present in a received signal is a function of the relative velocity of the receiver which received the signal with respect to the transmitter which transmitted the signal as well as the frequency (or frequency range) at which the signal was transmitted. Further, if a mobile communication unit is in motion, then a relative phase change will occur with each change in relative position of the mobile communication unit with respect to the central communication base site in the communication link. Furthermore, a fixed-position or slow moving mobile communication unit can experience variations in phase and carder frequency due to signal propagation-path-length changes in the communication channel.
Therefore, a need exists for a synchronization technique which is simple enough to be inexpensively built for use by low tier communication unit while at the same time providing rapid synchronization for use by a communication unit operating in the high tier communication system. The high tier communication system needs rapid synchronization, because the high tier communication system may utilize a spread spectrum communication process such as frequency hopping cede division multiple access (FH-CDMA) to communicate between a mobile communication unit and a central communication base site. In FH-CDMA systems, a receiving mobile communication unit, for example, may not know the hopping pattern used by the transmitting central communication base site prior to the start of transmission. Thus, the mobile communication unit must be able to synchronize within the time period of one frequency hop so that the mobile communication unit can determine the frequency of the next hop. In addition, it is desirable to design the synchronizing technique to minimize it's vulnerability to false correlation due to the presence of noise and/or interference in the communication channel.